The invention relates to an agent and method for use in connection with progenitor cells. More specifically, the invention relates to a protein capable of preserving progenitor cells and a method of using the protein for maintaining and preserving progenitor cells.
Each day the bone marrow generates and releases into the circulation several billion fully-differentiated, functional blood cells. Production of these cells derives from a small stock of quiescent progenitor cells (including the most primitive stem cells and other less primitive but still immature progenitors) by a process called hematopoiesis (Zipori 1992). The most primitive stem cells have the capacity to generate &gt;10.sup.13 cells containing all blood lineages (Turhan et al. 1989). The production of such a large number of cells is achieved by extensive proliferation coupled with successive differentiation steps leading to a balanced production of mature cells. Progenitor cells progressively lose their capacity to generate multiple cells lineages and eventually produce cells of one or two cell lineages.
Soluble regulators and cell-cell interactions mediate differentiation directions of immature progenitors through a tightly-controlled but inadequately understood process. Several of the body's soluble factors have been isolated and characterized both in culture and in animals (see, e.g., Ogawa (1993) and references therein). Regulators such as the colony stimulating factors (e.g., IL3, GM-CSF, G-CSF, M-CSF) not only induce proliferation and differentiation of progenitors capable of producing cells of either multiple cell lineages (IL3 and GM-CSF) or single cell lineages (G-CSF and M-CSF), but also maintain viability of their respective progenitors. Other regulators such as interleukin-1 (IL1), the kit ligand (KL), and thrombopoietin (Borge et al. 1996) increase viability of multipotential progenitors in addition to other functions. No known cytokines alone or in combination can maintain viability of primitive progenitors in liquid culture without stromal support beyond a few days.
Regulation of primitive stem cells appears to differ from that of immature, multilineage progenitors. Stem cells are primarily quiescent and do not appear to respond immediately to regulators that induce proliferation and differentiation. Maintenance of these cells in the body is mediated via cell-cell interactions and soluble regulators. Maintenance of quiescent stem cells in vitro has been achieved by culturing cells on adherent stromal layers with soluble regulators such as IL3, IL6, KL and LIF (Young et al. 1996). Recently, the addition of FL to this complex culture has been found to extend maintenance of quiescent stem cells from a few weeks to three months (Shah et al. 1996). Establishing stromal cells cultures is not easily applicable to clinical settings.
Lectins, defined as carbohydrate-binding proteins other than antibodies or enzymes, (Baronedes 1988), are widespread among plants, prokaryotes, and eukaryotes. Each lectin recognizes a specific carbohydrate moiety, and forms a non-covalent bond with the carbohydrate through a stereochemical fit of complementary molecules (e.g., hydrophobic pocket). Carbohydrates are widely present on cell surfaces (in the forms of glycoproteins, glycolipids, and polysaccharides), and appear to mediate cell-cell contacts including cell recognition (Sharon et al. 1989). Abnormal glycosylation patterns are associated with disease by causing alterations in a protein's conformation, stability, or protease resistance (Dwek 1995).
Gowda et al. (1994) describe the isolation of a mannose-glucose-specific lectin from the hyacinth bean (Dolichos lab lab). Purification and sequencing of this lectin is said to indicate that the protein includes two nonidentical subunits. The Gowda et al. publication describes evolutionary relationships of the lectin to other lectins, but does not ascribe any function to the protein beyond saccharide-binding in the plant source.
Cell agglutinating properties of certain plant lectins have been known for over 100 years. Certain lectins have been used as tools in immunology laboratories as potent, specific activators of T lymphocytes (phytohemagglutinin (PHA) and concanavalin A (ConA)) and B lymphocytes (pokeweed mitogen (PWM)) for over 30 years (Sharon et al. 1989). Some lectins have also been used to isolate hematopoietic progenitors for over 15 years (Gabius 1994a). Large numbers of cancer patients in Europe have received mistletoe lectin (Viscum album) intravenously as a candidate cancer therapy without major complications (Gabius 1994b). Whether these plant lectins act on mammalian cells via de novo means, or simply mimic their functional mammalian homologs is not yet known. No lectin has yet been successfully developed as a human therapeutic.
In view of the above considerations, it is clear that regulation of the hematopoietic process remains incompletely understood. Most soluble regulators identified, such as the colony stimulating factors and interleukins, induce proliferation and differentiation of progenitors cells in culture and their levels in the blood circulation increase during times of hematopoietic stress (e.g., blood loss, infection). For example, U.S. Pat. No. 4,808,611 describes a method of using IL 1 and a colony stimulating factor to induce proliferation and differentiation of hemopoietic stem cells. Some soluble regulators, such as IL 1, IL6, KL, FL, and Tpo, appear to provide increase viability of stem cells without directly affecting proliferation and differentiation. But no known soluble regulators, alone or in combination, have yet been reported that enable maintenance and amplification of stem cells populations in vitro without stromal cells. As a consequence, numerous potential therapeutic approaches to diseases such as cancer and genetic blood diseases remain unavailable.
Accordingly, it is one of the purposes of this invention to overcome the above limitations in methods of regulating hematopoietic processes, by providing a factor and method of protecting, maintaining, and expanding hematopoietic progenitor cell populations. It is another purpose of the invention to provide means for protecting the integrity of the hematopoietic processes in vivo as an adjunct to therapeutic treatments related to cancer and other diseases which can otherwise adversely impact upon the hematopoietic system.